I. Field of the Invention
The present invention relates to the field of passive safety in structures, particularly aeronautical structures. Even more particularly, the present invention relates to a subfloor structure for fixed- or rotary-wing aircraft having innovative energy absorption characteristics and a new method for enhancing the passive safety of an aircraft in case of impact.
II. Related Art and Other Considerations
Research into the impact safety of aeronautical structures of helicopters and of flying machines in general originated from a series of observations and statistical analyses carried out from the 1960s and 1970s onwards. This research had the merit of demonstrating for the first time that the consequences of impacts of aircraft, particularly helicopters, on the ground could be limited, provided that action was taken to prevent clearly identified hazardous events, such as the development of fire, the penetration of large suspended masses into the cabin, the action of intense acceleration on the occupants and the intrusion of panels and other structural components into the passenger space.
The increasingly precise identification of these events has made it possible to issue detailed military and civil specifications which have a significant effect on the design philosophy relating to the latest generation of helicopters and those of future generations. These specifications enable the nature of the problem to be defined unambiguously and clearly state the objectives to be attained.
If the requirements of “crashworthiness” (impact safety, or more generally passive safety) are to be met, research and development must be broadened in scope and become more detailed, the passive safety standards themselves must be progressively improved, and there must be a radical change in design philosophy. For example, a helicopter structure can have satisfactory crash behaviour only if the passive safety specifications are analysed at the preliminary design stage.
By way of example, the total energy absorption of a helicopter is obtained not only by exploiting the contributions offered, in sequence, by the handing gear, the lower part of the fuselage (“subfloor”), the seat with its supports and the roof with the transmission support structure. This is because the satisfactory crash behaviour of each of these components considered separately is not necessarily sufficient to ensure the satisfactory behaviour of the structure considered as a whole; it is also necessary to study and optimize the behaviour of all the elements taken together. The necessity of considering different aspects of the phenomenon simultaneously has led to the identification of new design systems.
The provision of an impact-resistant (“crash-resistant”) helicopter structure leads to increased costs, owing to the additional expense of a design process made more difficult by the constructional and technological complications and by the reduction of the useful volume and load capacity. On the other hand, the requirement to preserve passenger safety is of fundamental importance; it is therefore always necessary to seek structural arrangements and solutions which are efficient and effective in relation to energy absorption, and which can raise safety levels with a minimum increase in weight and cost.
Clearly, however, it is impossible to guarantee the survival of occupants in every type of accident, and therefore attention must be paid to the category of impact defined as “potentially survivable crashes”, in which the level and duration of stresses transmitted to the passengers do not exceed the limits of human tolerance.
Research has been carried out into the acceleration which a human being can withstand in an impact, and the results have shown that a human being can withstand horizontal accelerations of up to 45 g, but only for a very short period. These values decrease markedly if the acceleration acts in the vertical direction; in this case, the limit of tolerance becomes approximately 17 g for 0.04 seconds, falling to 5 g for 0.2 seconds.
All these data are fundamental to the design of structures capable of transmitting, in an impact, only those stresses which can be withstood by human beings, in order to ensure the survival of the latter. The object to be achieved is therefore that of designing and producing structures capable of being deformed to such an extent and in such a way as to limit the levels of acceleration to which the occupants of the aircraft are subjected, while ensuring the preservation of a survival cell.
These structures must be able to absorb energy by their controlled and programmed deformation. They must also preferably be positioned in the lower part of the fuselage of the aircraft or of the helicopter, since, in most cases, it is the lower part that impacts on the ground.
Among the other aspects of safety which increase the possibility of survival even after impact on the ground, the most important is unquestionably that of the prevention of fire and explosion and that of the reduction of the effects caused by these phenomena.
The cause of the development of a fire or an explosion in case of a crash is essentially the presence of the fuel system installed in the vehicle. The basic requirement is unquestionably the containment of the fuel. For this reason, the tanks and tubing of the fuel system must be strong and yet deformable; and they must be installed remotely from the systems or elements which may initiate a fire.
The present standards require that the tank, either full-scale or on a reduced scale, or portions of it, pass drop tests without showing any leakage. Each standard stipulates the main characteristics of this test, in other words the height of fall, the degree of filling of the tank, the presence of any fittings or of the structure of the aircraft which surrounds it. For example, some standards stipulate the complete filling of the tank and a height of fall of 20 m; others require 80% filling and a height of fall of 15.2 m.
In the present state of the art, the standards are applied in the military field only; in the civil field there are no known helicopters which meet the safety requirements concerning fire prevention.
It is clear from the above that, on the one hand, it is desirable to have a highly deformable subfloor structure which can absorb some of the energy in case of impact, while, on the other hand, it is necessary to have a fuel tank (generally housed in the subfloor area, in the case of a rotating wing aircraft for example) which is not deformed, in order to avoid highly dangerous deflagration.
Similarly, although air transport is considered to be safer than ground transport (whether by road or rail), it is highly vulnerable to terrorist attack and the risk that terrorists may place an explosive device on board an aircraft is always very high. The approach followed by designers up to the present has again been that of providing increasingly stiff and strong structures which can withstand explosions which may be caused intentionally or accidentally within the fuselage, particularly in its lower part or hold.
However, this approach conflicts with the requirement to provide deformable structures which absorb some of the energy developed in case of a crash or a bomb explosion. Moreover, the stiffening of a structure causes an inevitable and undesirable increase in weight, and in any case is generally insufficient to provide adequate protection against explosion.
U.S. Pat. No. 5,451,015 relates to an aeronautical composite structure with an integral fuel tank which tackles the problem of safety in case of impact.
U.S. Pat. No. 5,451,015 thus tackles a problem similar to that tackled by the present invention, but the solution is completely different, since, like the known solutions, it requires the stiffening of the structure.
U.S. Pat. No. 4,426,050 relates to tanks which can be released in advance in case of impact.
In other known solutions, the fuel is discharged if the pilot foresees a potential risk of crashing.
In the light of the prior art described above, the technology disclosed herein provides a structure, typically of the aeronautical type, which is locally yielding with high energy absorption and which also meets the requirements for passive safety of an aircraft.
The technology disclosed herein also provides a method for enhancing the passive safety of a structure, typically of the aeronautical type, with innovative and improved energy absorption characteristics.
The technology disclosed herein also provides a fuel tank, typically for aircraft, with innovative and improved energy absorption characteristics.
The technology disclosed herein comprises creation of suitably designed vent valves in the walls of a substantially closed aeronautical structure, particularly on the walls of a tank. In the case of a fuel tank, these valves enable the fluid to be released in impact conditions and to be transferred into appropriate lateral bags (or supplementary tanks), thus enabling the subfloor to act satisfactorily as an impact absorber, while simultaneously absorbing some of the impact energy which is transferred to the fluid. In the case of a fuselage within which an explosion occurs, the vent valves allow the exit of the energy developed in the explosion and prevent the explosion from causing breaches in the fuselage itself.
Thus technology disclosed herein operates on an entirely different and opposite principle to that followed up to the present, in other words that of having very strong structures. The technology disclosed herein, on the contrary, provides for substantially closed structures which, in case of an increase in pressure beyond a certain level, yield locally in predetermined positions in such a way as to reduce the pressure level.
The technology disclosed herein can conveniently be associated with conventional systems, such as safety valves, which can help to contain any fire, or at least to limit its extent, by cutting off the supply to the engines and isolating the tank.
In a first aspect, the technology disclosed herein provides a substantially closed structure, particularly for aeronautical applications, in which at least one of the walls of the structure comprises at least one vent valve which can be opened when the pressure inside the substantially closed structure exceeds a predetermined value, this predetermined value being at least twice (but preferably three times) the value of the normal operating pressure.
In a further aspect, the technology disclosed herein provides a method for enhancing the passive safety of a substantially closed structure, particularly for aeronautical applications, comprising the step of providing, in at least one wall of the structure, at least one vent valve which can be opened when the pressure inside the substantially closed structure exceeds a predetermined value, this predetermined value being at least twice (but preferably three times) the value of the normal operating pressure.
The technology disclosed herein is applicable to substantially closed and substantially rigid structures for aeronautical applications, for example fuel tanks (conveniently of the helicopter type) or fuselages.